[source] Add AGENT_VERSION variable in install
[source] Add AGENT_VERSION variable in install

[skip ci]
It will default to a fixed "latest tag" that we put manually as the
`DEFAULT_AGENT_VERSION` string in the code.
It's useful when you want to install the master's tip code or for
instance
if one wants to test his code on a separate brach before releasing it.

lookout/dd-agent,brettlangdon/dd-agent,brettlangdon/dd-agent,c960657/dd-agent,takus/dd-agent,brettlangdon/dd-agent,cberry777/dd-agent,Wattpad/dd-agent,pmav99/praktoras,cberry777/dd-agent,jyogi/purvar-agent,pmav99/praktoras,lookout/dd-agent,jyogi/purvar-agent,Wattpad/dd-agent,c960657/dd-agent,jyogi/purvar-agent,c960657/dd-agent,jyogi/purvar-agent,tebriel/dd-agent,brettlangdon/dd-agent,Wattpad/dd-agent,tebriel/dd-agent,indeedops/dd-agent,pmav99/praktoras,manolama/dd-agent,cberry777/dd-agent,indeedops/dd-agent,cberry777/dd-agent,lookout/dd-agent,c960657/dd-agent,manolama/dd-agent,takus/dd-agent,gphat/dd-agent,takus/dd-agent,indeedops/dd-agent,takus/dd-agent,gphat/dd-agent,brettlangdon/dd-agent,tebriel/dd-agent,pmav99/praktoras,manolama/dd-agent,tebriel/dd-agent,Wattpad/dd-agent,gphat/dd-agent,gphat/dd-agent,jyogi/purvar-agent,lookout/dd-agent,takus/dd-agent,manolama/dd-agent,pmav99/praktoras,lookout/dd-agent,gphat/dd-agent,indeedops/dd-agent,tebriel/dd-agent,c960657/dd-agent,Wattpad/dd-agent,cberry777/dd-agent,indeedops/dd-agent,manolama/dd-agent